Mechanical regulation of endocytosis by protein condensate capillary forces

Max Ferrin

Clathrin-mediated endocytosis (CME)

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(Lacy et al., 2018)

Mounting evidence of CME proteins forming liquid-like condensates

(Li et al., 2012; Bergeron-Sandoval et al., 2021; Day et al., 2021; Kozak and Kaksonen, 2022)

Condensates can deform membranes through capillary forces

(Kusumaatmaja et al., 2021)

How does capillarity influence endocytic membrane bending?

Two complementary modeling approaches

Toy geometrical model Continuum membrane mechanics model
everything is a spherical cap discretized shapes governed by local forces
can be solved analytically must be simulated numerically
relatively easy to write out and program complicated math and simulation techniques required
unrealistic membrane shapes and kinks realistic membrane curvature
purely equilibrium solution can simulate temporal dynamics

Mechanical influence of condensate in toy model

Condensate can provide net assistive force through wetting

Condensate surface tension results in energy barrier to initiation of membrane bending

The condensate can never be assistive to the flat-curved membrane transition

Droplet volume sets the energy barrier amplitude and position

Droplet volume sets the energy barrier amplitude and position

Summary: Condensate capillary forces inhibit membrane bending initiation, but assist completion

Aside: Different behavior in constant coat area vs. constant curvature alternative models

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Aside: Free energy landscape of constant coat area vs. constant curvature alternative models

Validation of toy model results with continuum mechanics model

Toy geometrical model Continuum membrane mechanics model

Supplement: coated membrane stabilizes at predicted curvature

Supplement: condensate stabilizes at predicted contact angle

Results in progress: complementary simulations to toy model

Model condensate stalls initiation of membrane bending at similar parameter regimes between models

Physiological significance of condensate mechanics

  • Under which parameter regimes is there a large vs. negligible effect from the condensate?
  • Which regime do experimental measurements place an endocytic condensate?
  • What parameters of the model might change over time in situ that could serve as regulatory features?

Results in progress: analysis of condensate's competition with coat bending energy

Hypothetical panel: comprehensive analysis of condensate impact on CME energy

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Hypothetical panels: experimental estimation of condensate model parameters

(Brangwynne et al., 2009; Kozak and Kaksonen, 2022)

Hypothetical panel: regulatory potential of condensate mechanics

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Thank you!

Summary: Condensate capillary forces inhibit membrane bending initiation, but assist completion

References

Bergeron-Sandoval, L.-P. et al. (2021) “Endocytic proteins with prion-like domains form viscoelastic condensates that enable membrane remodeling,” Proceedings of the national academy of sciences, 118(50). Available at: https://doi.org/10.1073/pnas.2113789118.
Brangwynne, C.P. et al. (2009) “Germline P Granules Are Liquid Droplets That Localize by Controlled Dissolution/Condensation,” Science, 324(5935), pp. 1729–1732. Available at: https://doi.org/10.1126/science.1172046.
Day, K.J. et al. (2021) “Liquid-like protein interactions catalyse assembly of endocytic vesicles,” Nature cell biology, 23(4), pp. 366–376. Available at: https://doi.org/10.1038/s41556-021-00646-5.
Kozak, M. and Kaksonen, M. (2022) “Condensation of Ede1 promotes the initiation of endocytosis,” Elife. Edited by M.I. Geli, 11, p. e72865. Available at: https://doi.org/10.7554/eLife.72865.
Kusumaatmaja, H. et al. (2021) “Wetting of phase-separated droplets on plant vacuole membranes leads to a competition between tonoplast budding and nanotube formation,” Proceedings of the national academy of sciences, 118(36), p. e2024109118. Available at: https://doi.org/10.1073/pnas.2024109118.
Lacy, M.M. et al. (2018) “Molecular mechanisms of force production in clathrin-mediated endocytosis,” Febs letters, 592(21), pp. 3586–3605. Available at: https://doi.org/10.1002/1873-3468.13192.
Li, P. et al. (2012) “Phase transitions in the assembly of multivalent signalling proteins,” Nature, 483(7389), pp. 336–340. Available at: https://doi.org/10.1038/nature10879.